Drag ?

That's an easy one. Nobody knows. Period. Full stop.

You either need a wind tunnel or a very accurate way of determining the horse power you are using.

Neither one is very practical. So really, who cares? If you are worried about 25 cents worth of fuel one way or the other, just stay home.

Camping (RVing) is for fun. Cost (within reason) is secondary.

YMMV

Only way to find out for sure would be wind tunnel testing.

How would you ever know? Wind tunnel test? I'm no authority on this topic but I think it would be negligible and maybe not even measurable. I guess you could try checking mpg with it open compared to closed. There are a lot of other things that would impact MPG rather than your sliding bed cover.
The wing, however might help especially if your 5th wheel cap is rather flat. Mine is very rounded and curved and I doubt if a wing would reduce wind resistance very much.
I think slowing down to 55mph would have the greatest impact on fuel consumption.

Prior to buying a MH I asked a round about many things, including fuel consumption, the favorite response was from people like you who always replied "If you have to ask, you can't afford it " Yet those same people complain about a 15 cent nut and bolt. If you don't know what your average MPG is you have no idea if you have a problem developing, which can add up to a few thousand dollars in repairs.
Besides A good Driver works out MPG with pen and paper, as they do other costs, only and idiots drives around unaware of expenses. I'm sure you could learn a few economic solutions if you were to go grocery shopping with your other half !!!
And if you have money to throw away then I suggest you pick a worthwhile charity and make a monthly contribution.

Know of a place where you can do some uninterrupted, non-public testing.....like an abandoned air field, long lonely stretch of unused roadway etc.

Hook up 5th wheel..open bed, OEM tailgate
Get up to 70 mph then place truck in neutral and let it coast to a stop....time/measure distance

Repeat with open tailgate
Repeat with louvered tailgate
Repeat with tonneau cover partial closed
Repeat with windows open


Oh never mind. Go camping and don't make RVng so complicated.

IBcarguy wrote:
How would you ever know? Wind tunnel test? I'm no authority on this topic but I think it would be negligible and maybe not even measurable. I guess you could try checking mpg with it open compared to closed. There are a lot of other things that would impact MPG rather than your sliding bed cover.
The wing, however might help especially if your 5th wheel cap is rather flat. Mine is very rounded and curved and I doubt if a wing would reduce wind resistance very much.
I think slowing down to 55mph would have the greatest impact on fuel consumption.

I appreciate the reply, and fully understand your point, however a study at Institute of Fluid Science Tohoku University 2-1-1, Katahira, Aoba-Ku, Sendai JAPAN, Using a wind tunnel showed that a short bed truck had little effect on drag and a large bed truck created a lot of drag while and driving with the tailgate down was even worse for both vehicles. The Geometry of the Truck Bed also made a difference, but as most truck beds in the USA are within the same geometry field I don't see a big comparative difference.
I'm my scenario, I am in essence reducing a long bed to a short bed, but the area behind the cab to where it becomes a short bed is a question I don't have an answer for, nor does this study give any idea of how the drag is increased or decreased when a trailer is attached.
The wing, as has been stated many times does work, If it didn't the Commercial Industry would not be using it, Plus assuming all other aspects are in tune and maintained the distance between the wing and the trailer is the biggest determining factor of efficiency. I agree that a cap with a curve is more stream lined than a brick wall cap, and there are caps with contorted shapes creating a stream line effect. But again the distance between the back of the truck and the cap on the trailer is an unknown..
In theory it should reduce drag I guess trial and error will provide the answer...

Thanks

Myth busters tested tailgate up, tailgate down and tonneau cover. Up got the best mpg...something to do with lift caused by the vortex of air circulating within the bed.

Not sure how having a 5er attached would change the outcome.

Impulse24 wrote:
I appreciate the reply, and fully understand your point, however a study at Institute of Fluid Science Tohoku University 2-1-1, Katahira, Aoba-Ku, Sendai JAPAN, Using a wind tunnel showed that a short bed truck had little effect on drag and a large bed truck created a lot of drag while and driving with the tailgate down was even worse for both vehicles. The Geometry of the Truck Bed also made a difference, but as most truck beds in the USA are within the same geometry field I don't see a big comparative difference.
I'm my scenario, I am in essence reducing a long bed to a short bed, but the area behind the cab to where it becomes a short bed is a question I don't have an answer for, nor does this study give any idea of how the drag is increased or decreased when a trailer is attached.
The wing, as has been stated many times does work, If it didn't the Commercial Industry would not be using it, Plus assuming all other aspects are in tune and maintained the distance between the wing and the trailer is the biggest determining factor of efficiency. I agree that a cap with a curve is more stream lined than a brick wall cap, and there are caps with contorted shapes creating a stream line effect. But again the distance between the back of the truck and the cap on the trailer is an unknown..
In theory it should reduce drag I guess trial and error will provide the answer...

Thanks

I think you're confusing the wings you see on trucks pulling 5th wheels with the wings/streamliners seen on semis. The wings on pickup trucks are too far away from the front of the trailer to help much, if any, at all. The "wings" on semi trucks are almost touching the trailer and are of the same height. They will work, wings on pickups not so much.

I'm not confused at all, I have already stated the biggest problem with most wings are they are too far away from the trailer to be effective.
Another situation with a wing is very few are set to the right angle, causing a futile fit. May be just may be RZv'ers with wings should also incorporate a faring on each side of the truck to push the wind a way from the sides of the trailer!.
Again a little pen and paper will help you work out the ideal angle and hight and width required. If done right there will not be a bug line !!! But all this is getting away from the original question.